The present invention is related to control systems, and in particular to systems and methods for drift compensation in control circuits.
Electronic control circuits are used in many applications from motor controllers to frequency synthesizers. As just one example, phase lock loop circuits are used in many applications as frequency synthesizers to generate a precise frequency signal. An exemplary basic phase lock loop circuit is shown in FIG. 1, and may be used to manage clock frequency and phase. Turning to FIG. 1, a basic phase lock loop circuit 100 is shown that includes a phase/frequency detector 110, a loop filter 140 typically comprising some RC components, a voltage controlled oscillator 160, and a frequency divider 170. In operation, a reference frequency 180 is compared with a divided feedback of an output frequency 190, and the comparison is used to drive voltage controlled oscillator 160 to form output frequency 190 consistent with reference frequency 180. Where frequency divider 170 causes a division by ‘N’, phase lock loop circuit 100 forces output frequency 190 to be exactly N times reference frequency 180. Phase/frequency detector 110 delivers either positive or negative charge pulses to voltage controlled oscillator 160 depending upon whether the phase of output frequency 180 leads or lags reference frequency 190. The delivered charge pulses are integrated by loop filter 140 to generate a control voltage (VVCO) that is applied to voltage controlled oscillator 160 causing the frequency to increase or decrease. As will be appreciated from the preceding description, output frequency 190 may be adjusted by changing either or both of reference frequency 180 or the value of frequency divider 170.
For the purposes of this discussion, the operation of voltage controlled oscillator 160 is assumed to be linear as shown in FIG. 2. In particular, FIG. 2 shows a graph 200 with output frequency 190 plotted as a function of the control voltage applied to voltage controlled oscillator 160. A Y-Axis 220 represents frequency, and an X-Axis 210 represents voltage. The range of the control voltage is typically limited by an available power supply on an upper end 230 and by the control of the locking mechanism on a lower end 240, and this leads to a finite range between an upper or maximum frequency available and a lower or minimum frequency available. In some cases, it may-be that drift due to factors such as temperature, limits an effective frequency range of phase lock loop circuit 100. These same limitations affect the control of circuits other than phase lock loop circuits.
Hence, for at least the aforementioned reasons, there exists a need in the art for advanced systems and methods for drift compensation in a control circuit.